Project Summary The p53 gene family occupies central positions in stress response networks throughout the animal kingdom. In humans p53 is implicated in age-related diseases and altered in most human cancers. As transcription factors, p53 genes mediate selective activation and repression of targets to specify adaptive responses but, despite extensive characterization, precisely how p53 acts to suppress tumors is not well understood. Since p53 genes are broadly conserved, ancestral properties of these genes offer promising routes towards understanding functions of p53 that become deranged in human diseases. Toward this goal, we are exploring the p53 regulatory network in genetic models. These systems offer uniquely powerful opportunities for interrogating conserved networks that support human pathologies. For example, like mammalian counterparts, p53 genes in flies and fish specify adaptive responses to damage that preserve genome stability. Leveraging experimental tools that visualize real-time p53 action in vivo, we discovered that p53 normally restrains retrotransposons, which are mobile elements broadly implicated in sporadic and heritable human disease. We also showed that p53 genetically interacts with the piRNA pathway, an ancient and highly conserved pathway dedicated to the suppression of transposons in all animals. In addition, by exchanging the fly p53 gene with human p53 counterparts, we found that human p53 genes could similarly restrain transposons but mutated p53 alleles from cancer patients could not. These combined discoveries suggest that p53 acts through highly conserved mechanisms to repress transposons. Furthermore, since human p53 mutants are disabled for this activity, our findings raise the possibility that p53 mitigates cancer by suppressing the movement of transposons. Consistent with this, we uncovered preliminary evidence for unrestrained retrotransposons in p53 mutant mice and in p53-driven human cancers. This initiative will determine how p53 acts to contain mobile elements and inspects these properties in other human members of the p53 gene family. Within this framework, we also determine whether p53 mutations are permissive for cancer because they are permissive for deregulated transposons. Our approach integrates molecular systems in flies and zebrafish, together with models of p53-driven tumors in mice. Valuable insights emerging from this initiative may enable novel classifiers that permit us to stratify p53 alleles based on properties that antagonize mobile elements. Since p53 is firmly established in the etiology of cancers these insights may, in turn, deliver new biomarkers and inform new therapeutic opportunities.